In vivo and in vitro EPR oximetry with fusinite: a new coal-derived, particulate EPR probe

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

A Review of Low-Frequency EPR Technology for the Measurement of Brain pO2 and Oxidative Stress

EPR can uniquely measure paramagnetic species. Although commercial EPR was introduced in 1950s, the early studies were mostly restricted to chemicals in solution or cellular experiments using X-band EPR equipment. Due to its limited penetration (<1 mm), experiments with living animals were almost impossible. To overcome these difficulties, Swartz group, along with several other leaders in field, pioneered the technology of low frequency EPR (e.g., L-band, 1-2 GHz). The development of low frequency EPR and the associated probes have dramatically expanded the application of EPR technology into the biomedical research field, providing answers to important scientific questions by measuring specific parameters that are impossible or very difficult to obtain by other approaches. In this review, which is aimed at highlighting the seminal contribution from Swartz group over the last several decades, we will focus on the development of EPR technology that was designed to deal with the potential challenges arising from conducting EPR spectroscopy in living animals. The second half of the review will be concentrated on the application of low frequency EPR in measuring cerebral tissue pO₂ changes and oxidative stress in various physiological and pathophysiological conditions in the brain of animal disease models.

Oxygenic photosynthesis: EPR study of photosynthetic electron transport and oxygen-exchange, an overview

In this review, we consider the applications of electron paramagnetic resonance (EPR) methods to the study of the relationships between the electron transport and oxygen-exchange processes in photosynthetic systems of oxygenic type. One of the purposes of this article is to encourage scientists to use the advantageous EPR oximetry approaches to study oxygen-related electron transport processes in photosynthetic systems. The structural organization of the photosynthetic electron transfer chain and the EPR approaches to the measurements of molecular oxygen (O₂) with O₂-sensitive species (nitroxide spin labels and solid paramagnetic particles) are briefly reviewed. In solution, the collision of O₂ with spin probes causes the broadening of their EPR spectra and the reduction of their spin-lattice relaxation times. Based on these effects, tools for measuring O₂ concentration and O₂ diffusion in biological systems have been developed. These methods, named "spin-label oximetry," include not only nitroxide spin labels, but also other stable-free radicals with narrow EPR lines, as well as particulate probes with EPR spectra sensitive to molecular oxygen (lithium phthalocyanine, coals, and India ink). Applications of EPR approaches for measuring O₂ evolution and consumption are illustrated using examples of photosynthetic systems of oxygenic type, chloroplasts in situ (green leaves), and cyanobacteria.

Contribution of Harold M. Swartz to In Vivo EPR and EPR Dosimetry

In 2015, we are celebrating half a century of research in the application of Electron Paramagnetic Resonance (EPR) as a biodosimetry tool to evaluate the dose received by irradiated people. During the EPR Biodose 2015 meeting, a special session was organized to acknowledge the pioneering contribution of Harold M. (Hal) Swartz in the field. The article summarizes his main contribution in physiology and medicine. Four emerging themes have been pursued continuously along his career since its beginning: (1) radiation biology; (2) oxygen and oxidation; (3) measuring physiology in vivo; and (4) application of these measurements in clinical medicine. The common feature among all these different subjects has been the use of magnetic resonance techniques, especially EPR. In this article, you will find an impressionist portrait of Hal Swartz with the description of the 'making of' this pioneer, a time-line perspective on his career with the creation of three National Institutes of Health-funded EPR centers, a topic-oriented perspective on his career with a description of his major contributions to Science, his role as a mentor and his influence on his academic children, his active role as founder of scientific societies and organizer of scientific meetings, and the well-deserved international recognition received so far.

Optimizing hypoxia detection and treatment strategies

Clinical studies using Eppendorf needle sensors have invariably documented the resistance of hypoxic human tumors to therapy. These studies first documented the need for individual patient measurement of hypoxia, as hypoxia varied from tumor to tumor. Furthermore, hypoxia in sarcomas and cervical cancer leads to distant metastasis or local or regional spread, respectively. For various reasons, the field has moved away from direct needle sensor oxygen measurements to indirect assays (hypoxia-inducible factor-related changes and bioreductive metabolism) and the latter can be imaged noninvasively. Many of hypoxia's detrimental therapeutic effects are reversible in mice but little treatment improvement in hypoxic human tumors has been seen. The question is why? What factors cause human tumors to be refractory to antihypoxia strategies? We suggest the primary cause to be the complexity of hypoxia formation and its characteristics. Three basic types of hypoxia exist, encompassing various diffusional (distance from perfused vessel), temporal (on or off cycling), and perfusional (blood flow efficiency) limitations. Surprisingly, there is no current information on their relative prevalence in human tumors and even animal models. This is important because different hypoxia subtypes are predicted to require different diagnostic and therapeutic approaches, but the implications of this remain unknown. Even more challenging, no agreement exists for the best way to measure hypoxia. Some results even suggest that hypoxia is unlikely to be targetable therapeutically. In this review, the authors revisit various critical aspects of this field that are sometimes forgotten or misrepresented in the recent literature. As most current noninvasive imaging studies involve PET-isotope-labeled 2-nitroimidazoles, we emphasize key findings made in our studies using 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)acetamide (EF5) and F-18-labeled EF5. These show the importance of differentiating hypoxia subtypes, optimizing drug pharmacology, ensuring drug and isotope stability, identifying key biochemical and physiological variables in tumors, and suggesting therapeutic strategies that are most likely to succeed.

Clinical EPR: unique opportunities and some challenges

Electron paramagnetic resonance (EPR) spectroscopy has been well established as a viable technique for measurement of free radicals and oxygen in biological systems, from in vitro cellular systems to in vivo small animal models of disease. However, the use of EPR in human subjects in the clinical setting, although attractive for a variety of important applications such as oxygen measurement, is challenged with several factors including the need for instrumentation customized for human subjects, probe, and regulatory constraints. This article describes the rationale and development of the first clinical EPR systems for two important clinical applications, namely, measurement of tissue oxygen (oximetry) and radiation dose (dosimetry) in humans. The clinical spectrometers operate at 1.2 GHz frequency and use surface-loop resonators capable of providing topical measurements up to 1 cm depth in tissues. Tissue pO2 measurements can be carried out noninvasively and repeatedly after placement of an oxygen-sensitive paramagnetic material (currently India ink) at the site of interest. Our EPR dosimetry system is capable of measuring radiation-induced free radicals in the tooth of irradiated human subjects to determine the exposure dose. These developments offer potential opportunities for clinical dosimetry and oximetry, which include guiding therapy for individual patients with tumors or vascular disease by monitoring of tissue oxygenation. Further work is in progress to translate this unique technology to routine clinical practice.

Electron paramagnetic resonance monitoring of ischemia-induced myocardial oxygen depletion and acidosis in isolated rat hearts using soluble paramagnetic probes

A new low-field electron paramagnetic resonance approach for noninvasive measurements of myocardial oxygen tension and tissue acidity was developed. The approach was applied to monitor myocardial pO(2) and pH in a model of global no-flow ischemia (30 min) and reperfusion in isolated perfused rat hearts. The myocardial oxygen measurements were performed using deuterated Finland trityl radical probe. A rapid decrease in myocardial pO(2) from 160 mmHg to about 2 ± 1 mmHg was observed within the first minute of ischemia followed by incomplete restoration of pO(2) to 50 mmHg during 30 min of reperfusion. The lower oxygen concentration after ischemia was attributed to the 50% reduction in coronary flow after ischemia as a consequence of myocardial ischemia and reperfusion damage. Myocardial pH measurements using a specially designed imidazoline pH-sensitive nitroxide showed severe myocardial acidification to pH 6.25 during 30 min of ischemia. Preconditioning of the hearts with two 5-min periods of ischemia significantly reduced the acidification of myocardial tissue during sustained ischemia. Noninvasive electron paramagnetic resonance monitoring of myocardial oxygenation and pH may provide important insights into the mechanisms of ischemia and reperfusion injury and a background for development of new therapeutic approaches.

Hexamethyldisiloxane-based nanoprobes for (1) H MRI oximetry

Quantitative in vivo oximetry has been reported using (19) F MRI in conjunction with reporter molecules, such as perfluorocarbons, for tissue oxygenation (pO(2) ). Recently, hexamethyldisiloxane (HMDSO) has been proposed as a promising alternative reporter molecule for (1) H MRI-based measurement of pO(2) . To aid biocompatibility for potential systemic administration, we prepared various nanoemulsion formulations using a wide range of HMDSO volume fractions and HMDSO to surfactant ratios. Calibration curves (R(1) versus pO(2) ) for all emulsion formulations were found to be linear and similar to neat HMDSO for low surfactant concentrations (<10% v/v). A small temperature dependence in the calibration curves was observed, similar to previous reports on neat HMDSO, and was characterized to be approximately 1 Torr/ °C under hypoxic conditions. To demonstrate application in vivo, 100 µL of this nanoemulsion was administered to healthy rat thigh muscle (Fisher 344, n=6). Dynamic changes in mean thigh tissue pO(2) were measured using the PISTOL (proton imaging of siloxanes to map tissue oxygenation levels) technique in response to oxygen challenge. Changing the inhaled gas to oxygen for 30 min increased the mean pO(2) significantly (p<0.001) from 39 ± 7 to 275 ± 27 Torr. When the breathing gas was switched back to air, the tissue pO(2) decreased to a mean value of 45 ± 6 Torr, not significantly different from baseline (p>0.05), in 25 min. A first-order exponential fit to this part of the pO(2) data (i.e. after oxygen challenge) yielded an oxygen consumption-related kinetic parameter k=0.21 ± 0.04 min(-1) . These results demonstrate the feasibility of using HMDSO nanoemulsions as nanoprobes of pO(2) and their utility to assess oxygen dynamics in vivo, further developing quantitative (1) H MRI oximetry.

Nano-emulsions of fluorinated trityl radicals as sensors for EPR oximetry

This article reports the development and evaluation of two nano-emulsions (F45T-03/HFB and F15T-03/PFOB) containing fluorinated trityl radicals dissolved in perfluorocarbons. Preparation with a high-pressure homogenizer conferred sub-micronic size to both nano-emulsions. In vitro and in vivo EPR spectroscopy showed that the nano-emulsions had much greater oxygen sensitivity than the hydrophilic trityl, CT-03. In vivo experiments in rodents confirmed the ability of the nano-emulsions to follow the changes in oxygen concentration after induced ischemia. Histological evaluation of the tissue injected with the nano-emulsions revealed some acute toxicity for the F45T-03/HFB nano-emulsion but none for the F15T-03/PFOB nano-emulsion. These new formulations should be considered for further EPR oximetry experiments in pathophysiological situations where subtle changes in tissue oxygenation are expected.

Proton imaging of siloxanes to map tissue oxygenation levels (PISTOL): a tool for quantitative tissue oximetry

Hexamethyldisiloxane (HMDSO) has been identified as a sensitive proton NMR indicator of tissue oxygenation (pO(2)) based on spectroscopic spin-lattice relaxometry. A rapid MRI approach has now been designed, implemented, and tested. The technique, proton imaging of siloxanes to map tissue oxygenation levels (PISTOL), utilizes frequency-selective excitation of the HMDSO resonance and chemical-shift selective suppression of residual water signal to effectively eliminate water and fat signals and pulse-burst saturation recovery (1)H echo planar imaging to map T(1) of HMDSO and hence pO(2). PISTOL was used here to obtain maps of pO(2) in rat thigh muscle and Dunning prostate R3327 MAT-Lu tumor-implanted rats. Measurements were repeated to assess baseline stability and response to breathing of hyperoxic gas. Each pO(2) map was obtained in 3(1/2) min, facilitating dynamic measurements of response to oxygen intervention. Altering the inhaled gas to oxygen produced a significant increase in mean pO(2) from 55 Torr to 238 Torr in thigh muscle and a smaller, but significant, increase in mean pO(2) from 17 Torr to 78 Torr in MAT-Lu tumors. Thus, PISTOL enabled mapping of tissue pO(2) at multiple locations and dynamic changes in pO(2) in response to intervention. This new method offers a potentially valuable new tool to image pO(2) in vivo for any healthy or diseased state by (1)H MRI.

Molecular imaging of hypoxia

Hypoxia, a condition of insufficient O2 to support metabolism, occurs when the vascular supply is interrupted, as in stroke or myocardial infarction, or when a tumor outgrows its vascular supply. When otherwise healthy tissues lose their O2 supply acutely, the cells usually die, whereas when cells gradually become hypoxic, they adapt by up-regulating the production of numerous proteins that promote their survival. These proteins slow the rate of growth, switch the mitochondria to glycolysis, stimulate growth of new vasculature, inhibit apoptosis, and promote metastatic spread. The consequence of these changes is that patients with hypoxic tumors invariably experience poor outcome to treatment. This has led the molecular imaging community to develop assays for hypoxia in patients, including regional measurements from O2 electrodes placed under CT guidance, several nuclear medicine approaches with imaging agents that accumulate with an inverse relationship to O2, MRI methods that measure either oxygenation directly or lactate production as a consequence of hypoxia, and optical methods with NIR and bioluminescence. The advantages and disadvantages of these approaches are reviewed, along with the individual strategies for validating different imaging methods. Ultimately the proof of value is in the clinical performance to predict outcome, select an appropriate cohort of patients to benefit from a hypoxia-directed treatment, or plan radiation fields that result in better local control. Hypoxia imaging in support of molecular medicine has become an important success story over the last decade and provides a model and some important lessons for development of new molecular imaging probes or techniques.

Novel1H NMR approach to quantitative tissue oximetry using hexamethyldisiloxane

19F NMR spin-lattice relaxometry of hexafluorobenzene (HFB) has been shown to be a highly sensitive indicator of tumor oxygenation. In this study hexamethyldisiloxane (HMDSO) was identified as a proton NMR analog, and its potential as a probe for investigating dynamic changes in tissue oxygen tension (pO2) was evaluated. HMDSO has a single proton resonance (delta= -0.3 ppm) and the spin-lattice relaxation rate, Rl (= 1/T1) exhibits a linear dependence on pO2: R1 (s(-1)) = 0.1126 + 0.0013* pO2 (torr) at 37 degrees C. To demonstrate application in vivo, HMDSO was administered into healthy rat thigh muscle (100 microl) and tumors (50 microl). Local pO2 was determined by using pulse-burst saturation recovery (PBSR) 1H NMR spectroscopy to assess R1. Water and fat signals were effectively suppressed by frequency-selective excitation of the HMDSO resonance. Rat thigh muscle had a mean baseline pO2 of 35 +/- 11 torr, with a typical stability of +/-3 torr over 20 min, when the rats breathed air. Altering the inhaled gas to oxygen produced a significant increase in pO2 to 100-200 torr. In tumors, altering the inspired gas also produced significant (albeit generally smaller) changes. This new pO2 reporter molecule offers a potentially valuable new tool for investigating pO2 in vivo.

Development and evaluation of biocompatible films of polytetrafluoroethylene polymers holding lithium phthalocyanine crystals for their use in EPR oximetry

Electron paramagnetic resonance (EPR) oximetry is a powerful technology that allows the monitoring of oxygenation in tissues. The measurement of tissue oxygenation can be achieved using lithium phthalocyanine (LiPc) crystals as oxygen reporters. In order to have biocompatibility for the sensing system and to assure long-term stability in the responsiveness of the system, we developed films of Teflon AF 2400 with embedded LiPc crystals. These systems can be used as retrievable inserts or parts of an implantable resonator or catheter. Atomic force microscopy studies revealed that the surface of the films was regular and planar. The response to oxygen of the sensor (EPR linewidth as a function of pO(2)) remained unchanged after implantation in mice, and was not affected by sterilization or irradiation. The use of resonators, holding LiPc embedded in Teflon AF 2400, implanted in the gastrocnemius muscle of rabbits allowed the monitoring of oxygen during several weeks. Several assays also demonstrated the biocompatibility of the system: (1) no hemolytic effect was noted; (2) no toxicity was found using the systemic injection test of extracts; (3) histological analysis in rabbit muscle in which the films were implanted for 1 week or 3 months was similar to standard polyethylene biocompatible devices. These advanced oxygen sensors are promising tools for future pre-clinical and clinical developments of EPR oximetry. These developments can be applied for other applications of biosensors where there is a need for oxygen permeable membranes.

Simultaneous NIR-EPR spectroscopy of rat brain oxygenation

Changes in cerebral oxygenation were simultaneously monitored by electric paramagnetic resonance (EPR) oximetry and near-infrared spectroscopy (NIRS). The tissue oxygen tension (t-pO2) was measured with an L-band (1.2 GHz) EPR spectrometer with an external loop resonator and the concentration of oxyhemoglobin [HbO2] and deoxyhemoglobin [Hb] were measured with a full-spectral NIRS system. Mean cerebral hemoglobin saturation (SmcO2) was calculated from the absolute [HbO2] and [Hb]. Six adult male rats were implanted with lithium phthalocyanine (LiPc) crystals into the left cerebral cortex. The change in oxygenation of the brain was induced by altering the inspired oxygen fraction (FiO2) in air from 0.30 at baseline to 0.0, 0.05, 0.10, and 0.15 for 1, 2, 5, and 5 minutes, respectively, followed by reoxygenation with an FiO2 = 0.30. Although both t-pO2 and SmcO2 values showed a decrease during reduced FiO2 followed by recovery on reoxygenation, it was found that SmcO2 recovered more rapidly than t-PO2 during the recovery phase. The recovery of t-pO2 is not only related to blood oxygenation, but also to delivery, consumption, and diffusion of oxygen into the tissue from the vascular system. Further studies will be required to determine the exact mechanisms for the delay between the recovery of SmcO2 and t-pO2.

In vivo measurement and imaging of tumor oxygenation using coembedded paramagnetic particulates

Tumor tissue oxygenation is an important parameter that is positively correlated to the chemo- or radiation treatment outcome of certain tumors. Hence, methods to accurately and noninvasively determine the concentration of oxygen (pO2) in tumors will be valuable. In this study, electron paramagnetic resonance (EPR) spectroscopy, utilizing microcrystalline particulates of lithium phthalocyanine (LiPc), was used to perform repeated measurements of pO2 as a function of tumor growth. We permanently embedded the particulates in the tumor by coimplanting them with RIF-1 tumor cells during inoculation in mice. This procedure enabled repeated measurements of oxygen concentration in the tumor to be obtained for >2 weeks during its growth phase. The particulates were stable and nontoxic to the tumor cells. Both an in vitro clonogenic assay and an in vivo tumor growth rate examination in C3H mice showed no apparent effect on cell proliferation or tumor growth rate. The measurements indicated that the pO2 of the tumor decreased exponentially with tumor growth and reached hypoxic levels ( approximately 4 mmHg) within 4 days after inoculation of the tumor cells. Spatial EPR imaging revealed a nonuniform distribution of the embedded particulates, which were localized mainly in the middle of the tumor volume. Oxygen mapping of the tumor, obtained by spectroscopic EPR imaging, showed significant variation of pO2 within the tumor. In summary, EPR spectroscopy and imaging with an embedded oximetry probe enabled accurate and repeated measurements of pO2 to be obtained in growing tumors under nonperturbing conditions.

Mechanism of oxygen-induced EPR line broadening in lithium phthalocyanine microcrystals

EPR oximetry has been recognized as an important tool for determining oxygen concentration in biological tissues, in vivo. The method relies on the use of oxygen-sensitive paramagnetic probes whose linewidth varies predictably, mostly linear, with oxygen concentration. Lithium phthalocyanine (LiPc) radical has emerged as the probe of choice due to its superior EPR sensitivity, oxygen response, and biocompatibility. However, there are certain limitations in the preparation of this material in a pure and usable form. In our efforts to improve the synthesis of this material for reliable use in oximetry applications, we developed microcrystalline particulates that showed several advantages over other probes. Despite its advantages, the probe shows linear response to pO2 only in the range of 0-70 mmHg, beyond which a saturation behavior is observed. The goal of this study was to understand the mechanism of the interaction of oxygen with LiPc in order to interpret the experimentally observed linewidths. We propose a dual-spin model in which the freely diffusing spins of LiPc are converted to fixed spins by adsorption of molecular oxygen. The proposed mechanism was verified from the effect of oxygenation/deoxygenation processes on the linewidth of LiPc. In summary, we demonstrated that adsorption of oxygen molecules on LiPc contributes to a nonlinear line-broadening effect. This understanding is important for the future design of new EPR oximetry probes.

EPR oximetry of tumors in vivo in cancer therapy

The partial oxygen pressure (pO2) in tumors is considered to be one of important factors that affect the response of tumors to different treatment. Therefore, we anticipate that the information about the variation of oxygen concentration in tumors can be used as a guide for individualizing radiotherapy, chemotherapy, and especially the combined therapies. There is thus a need to obtain quantitative data on the effects of different therapies on tumor oxygenation under in vivo conditions. One of the methods, which enable these measurements is EPR oximetry. In this work basic principles of the method will be described as well as some examples of tumor oxygenation changes after application of chemotherapeutic drugs (vinblastine, cisplatin, bleomycin) or electric pulses in combination with cisplatin or bleomycin to fibrosarcoma SA-1 tumors in mice. A paramagnetic probe, a char of Bubinga tree, was implanted into the tumor (center and periphery) and in the muscle or subcutis. EPR spectra line-width, which is proportional to oxygen concentration, was measured with time after the treatments. Tumor oxygenation was reduced for 58% of pretreatment value 1 h after intraperitoneal injection of 2.5 mg kg(-1) VLB and returned to pretreatment level within 24 h. Reduction in oxygenation of muscle and subcutis was much smaller and returned to pretreatment value faster as in tumors. With cisplatin (4 mg kg(-1)) and bleomicyn (1 mg kg(-1)) the reduction was less than 15%, but increases in combined therapy to 70%. Similar reduction was observed also with electric pulses alone (eight pulses, 1300 V cm(-1), 100 micros, 1 Hz) with fast recovery of 8h. After electrochemotherapy the recovery was slower and occurs only after 48 h. This study demonstrates that EPR oximetry is a sensitive method for monitoring changes in tissue oxygenation after different treatments, which may have implications in controlling side effects of therapy and in the planning of combined treatments.

Measurement of oxygen consumption in mouse aortic endothelial cells using a microparticulate oximetry probe

The purpose of this study was to determine the rate of oxygen consumption in mouse aortic endothelial cells (MAECs) and to determine the effect of a variety of inhibitors and stimulators of oxygen consumption measured by electron paramagnetic resonance (EPR) spectroscopy utilizing a new particulate oximetry probe. We have previously demonstrated that the octa-n-butoxy derivative of naphthalocyanine neutral radical (LiNc-BuO) enables accurate, precise, and reproducible measurements of pO(2) in cellular suspensions. In the current study, we carried out measurements to provide an accurate determination of pO(2) in small volume with less number of cells (20,000 cells) that has not been possible with other techniques. To establish the reliability of this method, agents such as menadione, lipopolysaccharide (LPS), potassium cyanide, rotenone, and diphenyleneiodonium chloride (DPI) were used to modulate the oxygen consumption rate in the cells. We observed an increase in oxygen consumption by the cells upon treatment with menadione and LPS, whereas treatment with cyanide, rotenone, and DPI inhibited oxygen consumption. This study clearly demonstrated the utilization of EPR spectrometry with LiNc-BuO probe for determination of oxygen concentration in cultured cells.

Carbon blacks as EPR sensors for localized measurements of tissue oxygenation

New electron paramagnetic resonance (EPR) oximetry probes were identified in the class of carbon black materials. These compounds exhibit very high oxygen sensitivity and favorable EPR characteristics for biological applications. At low pO(2), the linewidth is particularly sensitive to changes in oxygen tension (sensitivity of 750 mG/mmHg). The application of the probes for oximetry was demonstrated in vivo: the pO(2) was measured in muscle in which the blood flow was temporarily restricted as well as in tumor-bearing mice during a carbogen breathing challenge. The responsiveness to pO(2) was stable in muscle for at least 3 months. No toxicity was observed using these materials in cellular experiments and in histological studies performed 2, 7, and 28 days after implantation. In view of their EPR characteristics (high sensitivity) as well as the well-characterized production procedure that make them available on a large scale, these probes can be considered as very promising tools for future developments in EPR oximetry.

Novel particulate spin probe for targeted determination of oxygen in cells and tissues

The synthesis and characterization of a new lithium octa-n-butoxy-substituted naphthalocyanine radical probe (LiNc-BuO) and its use in the determination of concentration of oxygen (oximetry) by electron paramagnetic resonance (EPR) spectroscopy are reported. The probe is synthesized as a needle-shaped microcrystalline particulate. The particulate shows a single-line EPR spectrum that is highly exchange-narrowed with a line-width of 210 mG. The EPR line-width is sensitive to molecular oxygen showing a linear relationship between the line-width and concentration of oxygen (pO(2)) with a sensitivity of 8.5 mG/mmHg. We studied a variety of physicochemical and biological properties of LiNc-BuO particulates to evaluate the suitability of the probe for in vivo oximetry. The probe is unaffected by biological oxidoreductants, stable in tissues for several months, and can be successfully internalized in cells. We used this probe to monitor changes in concentration of oxygen in the normal muscle and RIF-1 tumor tissue of mice as a function of tumor growth. The data showed a rapid decrease in the tumor pO(2) with increase of tumor volume. Human arterial smooth muscle cells, upon internalization of the LiNc-BuO probe, showed a marked oxygen gradient across the cell membrane. In summary, the newly synthesized octa-n-butoxy derivative of lithium naphthalocyanine has unique properties that are useful for determining oxygen concentration in chemical and biological systems by EPR spectroscopy and also for magnetic tagging of cells.

In vivo measurement of regional oxygenation and imaging of redox status in RIF-1 murine tumor: effect of carbogen-breathing

The purpose of this study was to noninvasively monitor tumor oxygenation and redox status during hyperoxygenation treatment, such as carbogen-breathing, in a murine tumor model using in vivo electron paramagnetic resonance (EPR) spectroscopy and imaging techniques. The study was performed using implanted lithium phthalocyanine (LiPc) microcrystals as the oximetry probe and 3-carbamoylproxyl (3-CP) as the redox probe in RIF-1 tumors implanted in the upper hind leg of C3H mice. Repetitive measurements of pO(2) from the same tumors as a function of tumor growth (8-24 mm in size) showed that the tumors were hypoxic and that the tumor pO(2) values were decreasing with tumor growth. Carbogen-breathing mostly showed an increase in the tumor oxygenation, although there were considerable variations in the magnitude of change among the tumors. The pharmacokinetic studies with 3-CP showed a significant decrease in the overall tumor reduction status in the carbogen-breathing mice. Spatially resolved (imaging) pharmacokinetic data over the tumor volume were obtained to visualize the distribution of the redox status within the tumor. The redox images of the tumor in the air-breathing mice showed significant heterogeneity in the magnitude and spatial distribution of reducing equivalents. On carbogen-breathing the tissue reduction status decreased considerably, with a concomitant decrease in the heterogeneity of distribution of the redox status. The results suggest that 1) carbogen-breathing considerably enhances tissue oxygenation and significantly decreases the redox status in RIF-1 tumor, and 2) changes in the magnitude and distribution of the redox status within the tumor volume during carbogen-breathing are correlated with the increased tissue oxygenation.

A naphthalocyanine-based EPR probe for localized measurements of tissue oxygenation

A new electron paramagnetic resonance (EPR) oximetry probe, based on a naphthalocyanine macrocycle, is reported to exhibit high oxygen sensitivity and favorable EPR characteristics for biological applications. The free radical probe, lithium naphthalocyanine (LiNc), is synthesized as fine microcrystalline powder with particle size less than 1 microm and high spin density. It exhibits a single sharp EPR peak, whose width varies linearly with oxygen partial pressure (pO2). The EPR spectrum is nonsaturable at typical microwave power levels (< 25 mW at X-band). These unique characteristics make this probe ideal for measuring oxygen concentration in biological tissues, in vivo. The peak-to-peak width under anoxic conditions is 0.51 G (at X-band), and it increases linearly with increase in oxygen partial pressure and reaches 26.0 G for 100% oxygen (760 mmHg), showing an oxygen sensitivity of 34 mG/mmHg. The probe responds to changes in pO2 quickly and reproducibly, thus enabling dynamic measurements of regional oxygenation in real time. The application of this probe for oximetry is demonstrated in an in vivo biological system. The changes in pO2 were monitored in the leg muscle tissue of a living mouse breathing room air and carbogen (95% oxygen + 5% CO2), alternatively. The mean pO2 measured with this probe in muscle tissues was consistent with values reported previously using other methods. Overall, the probe shows very desirable characteristics for localized measurements of tissue oxygenation.

Microencapsulation of carbon particles used as oxygen sensors in EPR oximetry to stabilize their responsiveness to oxygen in vitro and in vivo

The electron paramagnetic resonance (EPR) spectra of some paramagnetic materials exhibit a pO2 (partial pressure of oxygen)-dependent linewidth. By recording the EPR linewidth in vivo using low-frequency EPR spectrometers, it is possible to measure the partial pressure of oxygen in tissues. It has been found, however, that some of the paramagnetic materials with optimal spectroscopic properties in vitro may lose or change their responsiveness to oxygen in tissues. The aim of this study was to microencapsulate paramagnetic particles by biopolymers in order to stabilize their responsiveness to oxygen. Carbohydrate char particles (Bubinga) were encapsulated with different biopolymers: cellulose acetate or cellulose triacetate, silicone and polyurethane. The performance of the materials was evaluated in vitro and in vivo. X-band EPR spectroscopy was used to test the variation of the calibration curve (EPR linewidth as a function of the pO2) after incubation in saline and after prolonged residence in tissues. The stability of the responsiveness to PO2 in vivo was carried out by L-band EPR spectroscopy using mice that received injection of the oxygen sensors in the muscles. After residence in saline and prolonged residence in tissues, only the calibration curve of the silicone-coated (coating weight of 0.5% (w/w)) paramagnetic materials remained unchanged, while those of oxygen sensors coated with cellulose acetate, cellulose triacetate and polyurethane changed.

High spatial resolution multi-site EPR oximetry. The use of convolution-based fitting method

We describe a new method to enhance the spatial resolution of multi-site electron paramagnetic resonance (EPR) oximetry. The method is suitable for any shape (density distribution function) of a solid paramagnetic material implanted in tissue. It corrects distortions of lineshapes caused by the gradient and thus overcomes limitations of previous multi-site EPR oximetry methods that restricted the ratio of the particle size to the distance between sites. The new method is based on consecutive applications of magnetic field gradients with the same direction but with a different magnitude and uses a convolution-based fitting algorithm to derive Lorentzian EPR linewidths of each individual peak of the EPR spectrum. The method is applicable for any particulate EPR oxygen sensitive materials whose EPR spectra can be approximated by a Lorentzian function or a superposition of Lorentzian functions. By incorporating this model of the lineshape in the data processing, we are able to decrease significantly the number of parameters needed for the calculations and to recover the oxygen concentration, even from quite noisy spectra. We (i) describe our method and the data-processing algorithm, (ii) demonstrate our approach in model and in vivo experiments, and (iii) discuss the limitations.

Development of biocompatible oxygen-permeable films holding paramagnetic carbon particles: evaluation of their performance and stability in EPR oximetry

EPR oximetry using paramagnetic particles relies on the measurement of the EPR linewidth, which is directly related to the pO2. It was previously found that some of the paramagnetic materials with optimal EPR spectroscopic properties in vitro may lose their responsiveness to oxygen in tissues (change of the calibration curve of the EPR linewidth as a function of the pO2). We hypothesized that coating paramagnetic particle materials could improve the stability of response, as well as the biocompatibility. In this study, very thin films holding paramagnetic materials were prepared with different biopolymers (cellulose acetate, cellulose triacetate, cellulose nitrate, silicone, and polyurethane) that already are accepted for clinical applications. Their performance was evaluated in EPR oximetry by measuring the stability of the calibration curves (EPR linewidth as a function of pO2) after a prolonged period in an aqueous environment (1 week in saline) or in vivo (implantation for 3 weeks under the skin of mice). We found that one type of silicone film was able to stabilize the responsiveness of an intrinsically unstable carbon material (a wood char).

Pathophysiological significance of in vivo ESR signal decay in brain damage caused by X-irradiation. Radiation effect on nitroxyl decay of a lipophilic spin probe in the head region

X-irradiation of mice decreased the decay rate of the in vivo ESR signal in the head region to 75% of the control when 3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-yloxy (MCPROXYL), a lipophilic and blood-brain barrier-permeable spin probe, was used. We attempted to identify the specific factor responsible for the decrease in the signal decay rate caused by X-irradiation. The signal decay of MCPROXYL in the head region depends on the following three factors: (1) blood concentration of MCPROXYL, (2) reduction to the corresponding hydroxylamine in the brain tissue, and (3) effusion of MCPROXYL from the brain tissue. Irradiation at 15 Gy did not significantly change the rate of decrease of blood concentration of MCPROXYL at 1 h post-irradiation. The reducing activity of the brain homogenate was not changed by the X-irradiation (15 Gy). The contents of MCPROXYL and its hydroxylamine derivative in the brain of 15 Gy-irradiated mice remained higher than in non-irradiated mice. These findings suggest that the effect of X-irradiation observed by in vivo ESR is attributable not to the redox reaction of MCPROXYL in the brain but to the change of the efflux rate of the MCPROXYL from the brain.

Accurate and sensitive measurements of pO(2) in vivo using low frequency EPR spectroscopy: how to confer biocompatibility to the oxygen sensors

Within the last few years, there has been a significant amount of progress using EPR oximetry, which has resulted in the availability of instrumentation and paramagnetic materials capable of measuring pO(2) in tissues with an accuracy and sensitivity comparable to or greater than that available by any other method. While the results obtained with EPR so far indicate that criteria for the measurements of pO(2)-such as accuracy, sensitivity, repeatability, and noninvasiveness-can be met, some of the paramagnetic materials with optimum spectroscopic properties (i.e., strong simple signals which are appropriately responsive to changes in pO(2)) may have some undesirable interactions with tissues, causing reactions with and/or losing responsiveness to oxygen. In this paper, several approaches are discussed, such as encapsulation procedures, which can result in the availability of oxygen-sensitive materials in a suitable configuration for long-term studies (absence of toxicity and preservation of the responsiveness to oxygen).

Microencapsulation of paramagnetic particles by pyrroxylin to preserve their responsiveness to oxygen when used as sensors for in vivo EPR oximetry

Using the broadening of the electron paramagentic resonance (EPR) linewidth of paramagnetic particles by oxygen, it is possible to make measurements of the partial pressure of oxygen in vivo. While the results obtained so far with EPR oximetry are very encouraging, several paramagnetic materials may lose their responsiveness to oxygen in tissues. This aim of this study was to provide evidence that an appropriate coating can preserve the oxygen sensitivity of paramagnetic materials in vivo. Two charcoals that have the oxygen-sensing properties required for EPR oximetry (combined with a tendency to lose responsiveness to oxygen when placed in tissues) were coated using pyroxylin. Sensitivity to variations in pO2 was checked by inducing hypoxia in the muscles of mice injected with charcoal. While the uncoated material lost responsiveness to oxygen within few days, the particles coated with 20-30% of pyroxylin did not lose their responsiveness for more than 2 months.

A computational model for tracking subsurface tissue deformation during stereotactic neurosurgery

Recent advances in the field of stereotactic neurosurgery have made it possible to coregister preoperative computed tomography (CT) and magnetic resonance (MR) images with instrument locations in the operating field. However, accounting for intraoperative movement of brain tissue remains a challenging problem. While intraoperative CT and MR scanners record concurrent tissue motion, there is motivation to develop methodologies which would be significantly lower in cost and more widely available. The approach we present is a computational model of brain tissue deformation that could be used in conjunction with a limited amount of concurrently obtained operative data to estimate subsurface tissue motion. Specifically, we report on the initial development of a finite element model of brain tissue adapted from consolidation theory. Validations of the computational mathematics in two and three dimensions are shown with errors of 1%-2% for the discretizations used. Experience with the computational strategy for estimating surgically induced brain tissue motion in vivo is also presented. While the predicted tissue displacements differ from measured values by about 15%, they suggest that exploiting a physics-based computational framework for updating preoperative imaging databases during the course of surgery has considerable merit. However, additional model and computational developments are needed before this approach can become a clinical reality.

Developing in vivo EPR oximetry for clinical use

This paper describes the rationale for carrying out electron paramagnetic resonance (EPR) oximetry studies in human subjects in the clinical setting and the potential approaches and specific steps needed to make such studies feasible and useful. While the approach is described specifically for EPR oximetry, many of the principles may apply to the initial clinical uses of other techniques. The suggested operational approach is to have the initial applications occur in as clinically useful and simple a manner as possible, with the expectation that once the technique is introduced and accepted in the clinical setting, that more complex and/or more technically difficult applications will be able to be developed. The initial approach will be based on EPR spectroscopy at 1.2 GHz focusing on applications for which in vivo EPR provides a clearly useful approach to important clinical problems for which currently there is no good alternative method. The EPR measurements can be carried out non-invasively by measurements within 10 mm of the surface after the placement of the paramagnetic material at the site of interest, or by the placement of a needle/catheter in the site of interest for the required time period. The suggested initial clinical applications are guiding therapy for individual patients with tumors or vascular disease, by direct measurements of tissue pO2.

Carbon-centered radicals as oxygen sensors for in vivo electron paramagnetic resonance: screening for an optimal probe among commercially available charcoals

It is known that some charcoals possess paramagnetic centers with an electron paramagnetic resonance (EPR) linewidth which can be broadened by oxygen. In order to identify potential candidates as sensors for in vivo EPR oximetry, we carried out a systematic study among commercially available charcoals. A total of 34 charcoals were tested. The steps used for the screening were: (1) to check the presence of paramagnetic centers in the material; (2) to measure the EPR linewidth in nitrogen and in air on the dry material and on a aqueous suspension of particles; (3) to calibrate the oxygen sensitive materials (EPR linewidth vs. pO2); (4) to test the sensitivity and stability of the response to changes of pO2 in a simple model of hypoxia induced in mice. Seventeen charcoals contained paramagnetic centers detectable by low-frequency EPR (1.1 GHz). The EPR spectrum consist of one single line which is typical of carbon-centered radicals (g-factor approximately 2). Eight charcoals presented sufficient interesting EPR properties (linewidth in nitrogen < 0.1 mT, linewidth in air for an aqueous suspension of particles > 0.15 mT) to be further characterized in vivo. Only three charcoals presented a stable, reproducible, and sensitive response to pO2 for more than 2 months. These three coals should be considered as good candidates to be used as oxygen sensor using in vivo EPR spectroscopy.

Separation and enrichment of the active component of carbon based paramagnetic materials for use in EPR oximetry

Carbon based paramagnetic materials are frequently used for EPR oximetry, especially in vivo, but the EPR spectra of these materials often have more than one paramagnetic center and/or relatively low signal intensity. To determine whether the multi-components of carbon based materials could be separated and enriched in the active component, we used density gradient centrifugation to separate the materials into several fractions. We studied two types of coals, gloxy and Pocahontas, and found these materials to have large density distribution. The separated density fractions had very different EPR spectra and intensities. The active component from the coal material had a more homogeneous EPR signal and significantly increased EPR signal intensity, whereas for India ink, only slight changes were observed. This result can be very useful in the development of better probes for EPR oximetry.

Preparation and EPR studies of lithium phthalocyanine radical as an oxymetric probe

The electron paramagnetic resonance (EPR) spectrum of the paramagnetic center in solid lithium phthalocyanine, LiPc, exhibits a pO2 (partial pressure of oxygen)-dependent line width. The compound is insoluble in water and is not easily biodegradable and, therefore, is a useful spin probe for quantitative in vivo oxymetry. Because EPR spectrometry is potentially a useful technique to quantitatively obtain in vivo tissue pO2, such probes can be used to obtain physiological information. In this paper, a simple experimental procedure for the preparation of LiPc using potentiostatic electrochemical methods is described. The setup was relatively inexpensive and easy to implement. A constant potential ranging from 0.05 to 0.75 V versus Ag+/AgCl(s) was used for obtaining LiPc. The EPR spectral studies were carried out using spectrometers operating at X-band and at radiofrequency (RF) at different pO2 values to characterize the spectral response of these crystals. The results indicate that, depending on the electrolysis conditions, the products contain mixtures of crystals exhibiting pO2-sensitive and pO2-insensitive line widths. Electrolysis conditions are reported whereby the pO2-sensitive LiPc crystals were the predominant product. The influence of the working surface of the electrode and the electrolysis time on the yield were also evaluated. The crystals of LiPc were also studied using a time-domain RF EPR spectrometer. In time-domain EPR, the signals that survive beyond the spectrometer dead time are mainly the narrow lines corresponding to the pO2-sensitive crystals, whereas the signals arising from the pO2-insensitive component of LiPc were found not to survive beyond the spectrometer dead time. This signal survival makes the time-domain EPR method more sensitive for pO2 measurements using LiPc because the line width becomes very narrow at very low pO2 and, concomitantly, the relaxation time T2 longer, with no modulation or power saturation artifacts that are encountered as in the continuous wave (cw) mode. Further, minimal contributions from object motion in the spectral data obtained using time-domain methods make it an advantage for in vivo applications.

The measurement of oxygen in vivo using EPR techniques

The measurement of pO2 in vivo using EPR has some features which have already led to very useful applications and this approach is likely to have increasingly wide and effective use. It is based on the effect of oxygen on EPR spectra which provides a sensitive and accurate means to measure pO2 quantitatively. The development of oxygen-sensitive paramagnetic materials which are very stable, combined with instrumental developments, has been crucial to the in vivo applications of this technique. The physical basis and biological applications of in vivo EPR oximetry are reviewed, with particular emphasis on the use of EPR spectroscopy at 1 GHz using particulate paramagnetic materials for the repetitive and non-invasive measurement of pO2 in tissues. In vivo EPR has already produced some very useful results which have contributed significantly to solving important biological problems. The characteristics of EPR oximetry which appear to be especially useful are often complementary to existing techniques for measuring oxygen in tissues. These characteristics include the capability of making repeated measurements from the same site, high sensitivity to low levels of oxygen, and non-invasive options. The existing techniques are especially useful for studies in small animals, where the depth of measurements is not an overriding issue. In larger animals and potentially in human subjects, non-invasive techniques seem to be immediately applicable to study phenomena very near the surface (within 10 mm) while invasive techniques have some very promising uses. The clinical uses of EPR oximetry which seem especially promising and likely to be undertaken in the near future are long-term monitoring of the status and response to treatment of peripheral vascular disease and optimizing cancer therapy by enabling it to be modified on the basis of the pO2 measured in the tumour.

Small particles of fusinite and carbohydrate chars coated with aqueous soluble polymers: preparation and applications for in vivo EPR oximetry

The development of oxygen-sensitive paramagnetic materials is being pursued actively because of their potential applications in in vivo EPR oximetry. Among these materials, several charcoals and carbohydrate chars are of special interest because of their desirable EPR properties: high sensitivity of the EPR linewidth to the partial pressure of oxygen, simple EPR spectra, and high spin density. Their potential use in humans, however, is limited by the need to demonstrate that they will not lead to deleterious effects. A strategy was used to optimize the biocompatibility of the oxygen-sensitive materials by decreasing the size of the particles and coating them with suspending or surfactive agents such as arabic gum, poloxamer (Pluriol 6800), and polyvinylpyrrolidone. The coated particles of a carbohydrate char and fusinite were characterized in vitro for their size, stability, and pO2 sensitivity. The feasibility of performing pO2 measurement was examined in vivo by inducing ischemia in the gastrocnemius muscle of mice. The use of arabic gum for coating the fusinite particles preserved the pO2 sensitivity in vivo, whereas the other surfactive agents led to a loss of the pO2 sensitivity in vivo. Small particles of fusinite coated by arabic gum and intravenously administered to mice accumulated in the liver, whereas the uncoated fusinite was toxic when injected intravenously due to the large size and aggregation of the particles. Histological studies performed up to 6 months after the injection in muscles of mice did not indicate any toxicity from the materials used in the present study.

Electron paramagnetic resonance and dynamic nuclear polarization of char suspensions: surface science and oximetry

Carbon chars have been synthesized in our laboratory from a variety of starting materials, by means of a highly controlled pyrolysis technique. These chars exhibit electron paramagnetic resonance (EPR) line shapes which change with the local oxygen concentration in a reproducible and stable fashion; they can be calibrated and used for oximetry. Biological stability and low toxicity make chars good sensors for in vivo measurements. Scalar and dipolar interactions of water protons at the surfaces of chars may be utilized to produce dynamic nuclear polarization (DNP) of the 1H nuclear spin population in conjunction with electron Zeeman pumping. Low-frequency EPR, DNP and DNP-enhanced MRI all show promise as oximetry methods when used with carbon chars.